Retroreflective articles have the ability to return substantial quantities of incident light, which otherwise would be reflected elsewhere, back towards the light source. This unique ability has led to widespread use of retroreflective articles in a variety of applications relating to traffic and personal safety. For example, in the personal safety area, retroreflective articles have been used on clothing, primarily to enhance a pedestrian""s visibility to motor vehicle operators after dusk or under other low visibility conditions. They also have been attached to book-bags and backpacks for the same purpose. Retroreflective articles can be made from cube-corner structures, such as cube-corner elements and cube-corner shaped cavities.
Cube-corner elements are structures that resemble the corner of a roomxe2x80x94that is, they contain three generally perpendicular faces that meet at a point or an apex. Cube-corner cavities, on the other hand, are structures that are defined by a recessed area bounded by three generally perpendicular faces that meet at a point in a valley. In general, the cube-corner formation process, whether forming cube-corner elements or cube-corner shaped cavities, is referred to as xe2x80x9creplication.xe2x80x9d Numerous methods exist for making cube-corner retroreflective articles. For example, U.S. Pat. No 5,450,235 to Smith et al. discloses an extrusion method, where a stream of polymer melt is extruded into a microstructured mold containing a multitude of cube-corner shaped cavities. The resulting product is a microstructured article that has a plurality of cube-corner elements arranged in an array. U.S. Pat. No. 5,691,846 to Benson, Jr. et al. discloses another method, where a curable resin solution is applied to a mold having a multitude of cube-corner shaped cavities. An overlay film is allowed to contact the resin. The resin solution is cured using actinic radiation to form a composite sheeting that has an array of cube-corner elements bonded to the overlay film.
Cube-corner shaped cavities have been made by various methods. Such methods, however, typically have separated the process of making a retroreflective article from the process of process of metalizing the articlexe2x80x94that is, the process of applying a reflective layer onto the article.
For example, U.S. Pat. No. 3,712,706 to Stamm discloses a reflective surface having an array of cube-corner shaped cavities. Stamm discloses a method where a cube-corner structure is made by inscribing three parallel equidistant intersecting grooves on a flat surface of a metal plate. This inscribing process forms a continuous pattern of sharp pointed, solid triagonal pyramids with 90xc2x0 internal dihedral angles. A dihedral angle is the angle formed by two intersecting planes. Stamm explains that the pyramid pattern is a negative form of the desired cube-corner shaped cavity pattern. The negative form (i.e., the pyramid pattern) may be used as a die for impressing the desired array of cube-corner cavities in a flat plastic surface or in metal foil. The cavity array can be mirror coated on all faces with, for example, evaporated gold, aluminum, or chromium. The cube-corner cavities are filled with an optically transparent solid medium to complete the making of the retroreflector.
U. S. Pat. No. 4,127,693 to Lemelson discloses another method of making cube-corner shaped cavities. In one embodiment, a reflector device comprises a base member that is capable of being molded or embossed with a plurality of cavities, each cavity being shaped to define a cube-corner reflector. The cavities have walls that are composed of three or more triangular flat surfaces. In a separate step, a thin film of metal may be deposited against the cavity surfaces. A protective transparent sheet can be placed over the cavities.
The present invention provides a new method of making a retroreflective article, which method can allow the article to be formed and made reflective contemporaneously. In brief summary, the inventive method comprises: (a) providing a device that has a first structured surface; (b) applying a reflective layer on the first structured surface such that a first adhesion value is established between the reflective layer and the first structured surface, the reflective layer having an exposed surface; (c) applying a molding polymer to the exposed surface of the reflective layer such that a second adhesion value is established between the reflective layer and the molding polymer, the second adhesion value being higher than the first adhesion value; and (d) separating the device from the molding polymer to yield the retroreflective article having a second structured surface, the separation causing a transfer of the reflective layer from the first structured surface to the retroreflective article. The device can be a tool or a mold. The inventive method differs from known methods in that it can combine the process of forming the second structured surface and applying a reflective layer in essentially a single step. When the reflective layer used in the method includes aluminum, applicants discovered that a new article was produced that had aluminum microstructures that are predominantly amorphous on a first side (the side near or next to the second structured surface) and predominantly crystalline on a second side (the side furthest away from the second structured surface).
An advantage of the method of the present invention is that it can eliminate the need to form the reflective layer directly on the inventive article. Because of the high temperature and high vacuum processing conditions typically present in many vapor deposition processes, residual solvents or monomers contained in a polymeric tool or mold can sometimes outgas and can cause blisters and defects in the reflective coating. When reflective layers have been deposited directly onto a polymeric tool or mold, investigators have had to take care in selecting ones that are capable of withstanding the vapor deposition process conditions. The differences between a tool and a mold are defined in detail below. The method of the present invention, however, provides the advantage in that the reflective layer is not directly vapor deposited to the second structured surface of the inventive article and therefore may not significantly impose upon the types of material that may be selected to make the inventive retroreflective article.
The new retroreflective article of the invention comprises: (a) a structured surface that is configured for allowing the article to retroreflect incident light; and (b) an aluminum reflective layer disposed on the structured surface, the aluminum layer having first and second sides, the first side disposed towards the structured surface and lies opposite the second surface, wherein the first side has an aluminum microstructure in a predominantly amorphous arrangement.
Retroreflective articles of the invention, which possess an aluminum-containing reflector, differ from known retroreflective articles in regard to the arrangement of the aluminum atoms. Unlike the articles described in U.S. Pat. Nos. 3,712,706 and 4,127,693, the present inventive article includes an aluminum reflective layer where the aluminum atoms disposed towards the structured surface have a generally amorphous arrangement and the metal atoms away from the structured surface have a generally crystalline arrangement.